It was back in the '80s that gastronome Nicholas Kurti, a professor of physics at Oxford University, picked up a slice of lamb and a bottle of white wine and decided to test a commonly held belief. Can a marinade really penetrate and tenderize meat? After coloring the white wine blue so he could watch its progress, he poured it around a slice of lamb. After 36 hours, it had penetrated only 10 millimeters (.39 inch).

News of Kurti's discovery has been even slower to penetrate. Most of us still believe in the tenderizing effects of marinades (most commonly, combinations of acids, oils and flavorings). But this foodie and atomic scientist proved that although acids--vinegar, lemon juice and wine, for example--as well as certain fruit enzymes in papaya, pineapple, kiwi and fig sap do tenderize meat, the process is just too slow for normal use and the resulting change in texture is unpleasant. (When Kurti tried to speed things up by using a hypodermic needle filled with pineapple juice to inject a roast, the meat acquired a mushy texture.)

McGee advises against soaking meat for longer than two hours in an acid marinade. "Any longer than that and the meat will have a kind of mealy stuff on the surface. The structured meat tissue becomes tiny protein particles, fine for a pate but not what you want in a steak."

You can do low-acid marinades for long periods, but really, it's rather pointless when if you want that flavor, you can just drizzle a bit when the meat is done.
Salt is what ultimately makes the biggest difference in flavor and texture of meat.